When the body of a cabin or the like is placed and fixed on the frame of an automobile, vibration-preventing rubber devices are interposed between the frame and the body so as to absorb and dampen vibrations. Thus, vibrations transmitted from tires to the frame during operation of the automobile may be prevented from propagating to the body.
In general, vibrations originating during the operation of vehicles have high and medium frequencies. Therefore, vibration-preventing devices having comparatively small spring constants are usually used to prevent vibrations. Such vibration-preventing devices, however, do not satisfactorily dampen vibrations in a low-frequency region, such as, for example, vibrations attributable to undulations of the surface of a bad road.
Multiple mounting type vibration-preventing devices have been developed which not only absorb high- and medium-frequency vibrations but also dampen low-frequency vibrations.
A multiple mounting vibration-preventing device 1 is disclosed, for example, in the official gazette of Japanese Utility Model Registration Application No. 59-231236 and depicted in FIG. 7. A middle supporter 4 is sandwiched between an upper supporter 2 and a lower supporter 3. The supporters 2, 3 and 4 are fitted snugly on an inner cylinder 5, and are held therebetween by the inner cylinder 5 and a bolt 6. The outer peripheral part of the lower end of an upper vibration-preventing rubber member 8, attached to an upper plate 7, is caulked and sealingly fixed betwen the upper supporter 2 and the middle supporter 4, and the outer peripheral end of the upper plate 7 is caulked and sealingly fixed to an upper outer cylinder 9. The outer peripheral part of the upper end of a lower vibration-preventing rubber member 11, attached to a lower plate 10, is caulked and sealingly fixed between the lower supporter 3 and the middle supporter 4. The outer peripheral end of the lower plate 10 is caulked and sealingly fixed to a lower outer cylinder 12.
The flange portion 13 of the upper outer cylinder 9 and the flange portion 14 of the lower outer cylinder 12 are sealingly fixed by welding or the like. An upper damper liquid chamber 16 and a lower damper liquid chamber 17 are defined by a partition wall made of rubber 15 which is interposed between the middle supporter 4 and the upper outer cylinder 9. The upper damper liquid chamber 16 and the lower damper liquid chamber 17 communicate through an orifice 18 provided in the middle supporter 4.
The high-frequency and medium-frequency vibrations are absorbed by the vibration-preventing members 8 and 11, while the low-frequency vibrations are dampened by the flow of a damper liquid between the upper and lower damper liquid chambers 16 and 17. The low-frequency vibrations are carried by the relative up and down motion of the outer cylinders 9, 12 and the middle supporter 4, and are attenuated under the damping action of the orifice 18. The damper liquid is a noncompressible fluid such as a coolant.
The relationship between a frequency corresponding to a peak damping coefficient, namely, the resonance frequency f.sub.n of the damper liquid within the orifice 13, the volume modulus k.sub.1 of the upper damper liquid chamber 9, the volume modulus k.sub.2 of the lower damper liquid chamber 12, the aperture area S of the orifice 13, the length l of the orifice 13, and the specific gravity .rho. of the damper liquid, is as follows: ##EQU1##
That is, the resonance frequency f.sub.n of the damper liquid may be lowered by reducing the aperture area S of the orifice 13, lowering the volume moduli k.sub.1 and k.sub.2 of the respective damper liquid chambers 9 and 12, or increasing the length l of the orifice 13.
In the prior-art vibration-preventing device, the outer peripheral part of the lower end of the upper vibration-preventing rubber member 8 is caulked between the upper supporter 2 and the middle supporter 4, while the outer peripheral part of the upper end of the lower vibration-preventing rubber member 11 is caulked between the lower supporter 3 and the middle supporter 4. The outer peripheral ends of the upper plate 7 and the lower plate 10 are caulked to the corresponding outer cylinders 9 and 12, respectively. Thus, it is necessary to caulk in four places. Such a structure is disadvantgeous because it is complicated and inefficient to assemble.
As stated above, the length l of the orifice 18 may be increased or the aperture area S thereof may be reduced to lower the resonance frequency f.sub.n of the damper liquid within the orifice 18. However, the length l has an upper limit based on the size of the vibration-preventing rubber device 1. Also, when the aperture area S is smaller than a predetermined value, flow resistance increases, and the peak value of the damping coefficient disadvantageously decreases.
To set the resonance frequency f.sub.n at a desired value, the volume moduli k.sub.1 and k.sub.2 of the respective damper liquid chambers 16 and 17 may be varied. However, the volume moduli k.sub.1 and k.sub.2 of the damper liquid in the damper liquid chambers 16 and 17 are determined by the volume moduli of the upper vibration-preventing rubber member 11, and the volume moduli of the vibration-preventing rubber members 8 and 11 is limited because these members 8 and 11 must support a static load applied from above to absorb the high-frequency and medium-frequency vibrations. It is therefore difficult to set the resonance frequency f.sub.n at the desired value.
As a result, in the prior-art vibration-preventing device 1, the damping function for the low-frequency vibrations is not satisfactory.